1. Field of Invention
The present invention relates to a high temperature furnace comprising insulation having a shaped thermocouple assembly port.
2. Description of the Related Art
Crystallization furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material in a crucible to produce an ingot. Production of an ingot from molten feedstock occurs in several specific temperature steps and temperature rates over many hours. For example, to produce a silicon ingot by the DSS method, silicon feedstock is added to a crucible, often contained in a graphite crucible box, placed into a DSS furnace and then heated to fully melt the feedstock. Typically, heating from room temperature to 1200° C. occurs over several hours at specified temperature rates before further heating for several hours up to 1550° C. The temperature, well above the silicon melting temperature of 1412° C., is maintained for several hours at this temperature to completely melt the silicon feedstock before applying a temperature gradient over 18 hours to directionally solidify the melt. The temperature is then reduced below the melting point of silicon to anneal the ingot over several hours, and, thereafter, the furnace is cooled down for several more hours before removing the formed ingot.
The challenge in consistently producing high quality ingots in a production scale furnace is to ensure that the melting, solidification, and annealing temperatures, temperature rates, and times can be accurately and consistently measured in the furnace hot zone where ingot formation occurs. For example, the hot zone of a furnace generally comprises a crucible containing feedstock and at least one heating element above or beside the crucible to melt the feedstock. Insulation typically surrounds at least the top and sides of the crucible and heating element(s) to contain the heat and define the hot zone, with one side of the insulation facing the furnace shell wall and the other side facing the heating element(s) within the hot zone. At least one thermocouple assembly, positioned through a thermocouple assembly port in the insulation and into the hot zone, is typically used in conjunction with a computer feedback mechanism to the heater power source to measure, control and maintain the correct temperature during various phases of ingot growth. Conventionally, the size or diameter of the port opening on the side of the insulation that faces the furnace shell is generally substantially equal to the size of the port opening on the side of the insulation that faces the heating element, creating a generally cylindrical port. The thermocouple assembly, typically comprising a thermocouple sensor encased in heat-protecting tubes housed in a protective sheath, for example, graphite, fits into and through this cylindrical port.
In an effort to grow ever larger ingots with existing production equipment, space within the hot zone must be maximized, often requiring that the heating elements reside in close proximity to the hot zone insulation. As such, thermocouple assembly(s) placed through conventional ports in the insulation to monitor process temperatures in the hot zone are typically positioned close to the heating element, for example, less than an inch. This close proximity places the thermocouple assembly in a large temperature gradient zone ranging from approximately 1500° C. at the heating element side of the insulation to approximately 1600-1700° C. at the heating element surface. This temperature gradient, typically spanning less than two inches from the insulation to the heating element surface, makes the placement of the thermocouple assembly highly susceptible to positional temperature measurement variability.
Difficulties in ensuring consistently repeatable heater control temperature measurements arise when a thermocouple assembly must be placed in a furnace at a specific distance relative to the heating element because the measured temperature is highly dependent on that distance. Therefore, the high positional sensitivity of the thermocouple assembly placed in a large temperature gradient zone can result in significant differences in the measured temperatures if not accurately placed in the designated position, requiring expensive and time consuming effort to measure and compensate for the observed differences.
As such, there is an increasing need in the industry for a simpler, more reliable, and cost effective means to position a thermocouple assembly in a high temperature furnace so that the resulting measured heater control temperature is substantially insensitive to thermocouple assembly position. The present invention reduces this high positional sensitivity and results in more consistent and repeatable heater control temperature measurements required for high quality ingot growth.